In recent years, there has been an increased awareness of the potential contamination of groundwater with nonaqueous phase liquids (NAPLs), which include, but are not limited to nonvolatile oils, alcohols, and volatile organic compounds (VOCs) such as benzene, toluene, xylene, perchloroethylene, and trichloroethylene. Many of these contaminants in groundwater supplies have originated from the excessive and widespread use of chlorinated hydrocarbons as degreasers, leaks from underground storage tanks, leachate from municipal and industrial landfill sites, or releases in industrial effluent streams. In light of the increasing amounts of contaminants, the U.S. Environmental Protection Agency (EPA) has established standards for the quality of drinking water. Moreover, methods have been developed to remove NAPLs and VOCs from underground water supplies before such supplies enter the water system. One such method is to pump the groundwater to the surface, treat it to remove from it any NAPLs, and then return the water to the aquifer. However, this method has met with only limited success.
An alternative method, Surfactant-Enhanced Aquifer Remediation (SEAR), is of increasing interest, especially where NAPLs are present in the ground water. In the SEAR process, an appropriately formulated aqueous solution of a suitable surfactant with or without alcohols or suitable polymers is pumped underground to mix with the contaminated groundwater. While underground, the surfactant solution forms micelles having a polar exterior and a hydrophobic interior, which trap VOCs, high boiling point oils, and polymers contaminating the groundwater. The micellar multicomponent aqueous solution is then collected and purified on the surface to recover the contaminants. Also, the micellar aqueous solution can then be used again to remove more VOCs from underground.
One of the principal methods used to purify the micellar aqueous solution is pervaporation, wherein the multicomponent aqueous solution to be purified is placed in contact with one side of a membrane, and the permeate is removed as a low pressure vapor from the other side of the membrane [Feng, X. And Huang, Y., Ind. Eng. Chem. Res., 36(4):1048-1066 (1997)]. The membrane may be symmetric or asymmetric in structure. Moreover, the membrane may be made of only one material, or more than one material. If the membrane is made of more than one material, it may be referred to as a "membrane composite."
In a conventional pervaporation process, the membrane composite may be in the form of a hollow fiber made of porous material having a bore, an inner surface, and an outer surface, and nonporous membrane disposed on the outer surface. A vacuum or sweep gas is used to lower the partial pressure in the bore of the fiber. The ulticomponent aqueous solution is then permitted to make contact with the nonporous membrane disposed on the outer surface of the fiber. The permeate then diffuses first through the nonporous membrane disposed on the outer surface of the hollow fiber, then through the porous membrane into the bore of the hollow fiber, where the permeate can be collected in vapor form.
However, the above known method has certain inherent limitations. For example, the volume of the hollow fiber bore is typically extremely small. Hence, when the permeate in vapor form reaches the hollow fiber bore, a pressure buildup may result which can reduce the driving force for pervaporation. Moreover, if the multicomponent aqueous solution contains some high-boiling point oils, then under the pressure difference applied across the membrane composite, these oils can permeate through the nonporous membrane and appear as a liquid throughout the porous membrane. Hence, the oil may clog the pores of the porous membrane, as well as the hollow fiber bore itself, thereby creating a pressure drop and drastically reducing the efficiency of the pervaporation process.
Another problem experienced with the above known method was the collection of a large volume of water because water being volatile, could dissolve into the membrane composite and permeate therethrough.
In an effort to overcome these problems, attempts have been made at a tube-side feed mode of operation, wherein the multicomponent aqueous solution contacts the inner nonporous inner surface of the hollow fiber membrane composite, and wherein the permeate diffuses to the outer surface for collection in an exit chamber defined by a shell surrounding the hollow fiber membrane composite. With this arrangement, a possible pressure buildup which could result in decreased pervaporation efficiency is avoided because of the increased volume of the exit chamber as compared with that of the hollow fiber bore.
However, this arrangement possesses a limitation in that there is a permeate side pressure drop in the porous membrane.